1. Field of the Invention
This invention relates to the solution mining of subterranean trona and/or nahcolite ore deposits with an aqueous sodium hydroxide solvent that is generated electrodialytically.
2. Description of the Prior Art
The use of aqueous sodium hydroxide (NaOH) for solution mining of subterranean trona ore and nahcolite ore deposits is known. U.S. Pat. No. 3,184,287 issued to Gancy describes a cyclic process for solution mining trona with aqueous sodium hydroxide, this solvent being regenerated by causticization of aqueous sodium carbonate with lime. U.S. Pat. No. 3,953,073 issued to Kube describes the use of aqueous sodium hydroxide for solution mining of trona and nahcolite, and of other NaHCO.sub.3 -containing ores, and discloses that the solvent requirements may be met either by causticization of soda ash with hydrated lime or by the electrolytic conversion of sodium chloride to sodium hydroxide.
Aqueous sodium hydroxide that is produced by the electrolysis of aqueous sodium chloride brines also results in the coproduction of chlorine gas and hydrogen gas, which must be disposed of or otherwise utilized. Electrodialysis of such aqueous brines, in a cell having permselective bipolar membranes, likewise produces aqueous sodium hydroxide, but with hydrogen chloride as a coproduct. An important advantage of electrodialysis over electrolysis is that the energy requirements for electrodialytically-generated sodium hydroxide is much less than that of electrolytically-generated sodium hydroxide.
U.S. Pat. No. 4,219,396 issued to Gancy et al. describes the operation of an electrodialysis cell in which soda ash is recovered using a brine feed. Aqueous sodium chloride is introduced as feed brine to a salt zone of the three-zone electrodialysis cell, aqueous hydrogen chloride is removed from an acid zone, and NaOH-enriched aqueous sodium carbonate is withdrawn from a base zone and carbonated with CO.sub.2 to convert the sodium hydroxide to sodium carbonate.
U.S. Pat. No. 4,391,680 issued to Mani et al. describes the operation of a two-compartment electrodialysis cell in which an aqueous brine feed is introduced into the first compartment. An acidified salt solution, containing HCl, is withdrawn from the first compartment for use as feed to an electrolytic cell, and aqueous sodium hydroxide is withdrawn from the second compartment.
U.S. Pat. No. 4,498,706 issued to Ilardi et al. for "Solution Mining of Trona or Nahcolite Ore with Aqueous NaOH and HCl Solvents," teaches the use of electrodialysis cell coproducts, hydrogen chloride and sodium hydroxide, as separate aqueous solvents in an integrated solution mining process for recovering soda ash. The electrodialytically-produced aqueous sodium hydroxide is utilized as the primary solution mining solvent and the co-produced aqueous hydrogen chloride is used to solution-mine NaCl-contaminated ore deposits to recover a brine feed for the electrodialysis cell operation.
Aqueous hydrogen chloride possesses some drawbacks as a solution mining solvent, which discourage its use in this application. The fact that aqueous hydrogen chloride is a strong acid may create environmental objections against its use in a subterranean ore deposit; complete neutralization of the residual acid, following the termination of solution mining activity, would be difficult and costly. Reaction of the hydrogen chloride solvent with the NaHCO.sub.3 -containing ore converts the ore's valuable alkali values into less desirable sodium chloride.
The solution mining method of the present invention avoids these drawbacks while nevertheless employing an electrodialysis cell to produce aqueous sodium hydroxide solvent. Aqueous sodium hydroxide is the sole mining solvent contacted with the underground ore deposits. Aqueous hydrogen chloride, coproduced in the electrodialysis cell, is not recovered but is instead completely reused in the electrodialysis cell operation so that it plays no role in the solution mining of the subterranean ore deposits.